The present invention relates to a method of diffusively bonding a metallic cladding to a metallic substrate; and in particular, a method of metallurgically bonding a tungsten carbide alloy cladding to a carbon steel substrate.
Surface instability of structural substrates is a significant problem in many advanced industrial and automotive applications. Highly corrosive environments are generated by the combustion process and when these highly corrosive environments are combined with high operating temperatures, limitations arise with respect to the materials which can be successfully used. Additionally, a further source of surface instability arises in high strength applications wherein protective claddings have been used in conjunction with otherwise suitable substrate to provide surface protection thereto. The bonding of a corrosion and wear resistant or high strength cladding to a substrate represents a solution to the surface instability problem.
There has been developed a process for accomplishing this desired bonding of two dissimilar metals which is known as hot isostatic pressing (HIPing) process. The hot isostatic pressing process involves the simultaneous application of pressure and temperature to a workpiece. In essence, the workpiece is squeezed from all sides at elevated temperatures. Generally, a pressure, up to approximately 30,000 psi, is applied by a pressure or energy transmitting medium; i.e. gas or molten inert glass powder or beads. The applied pressure, along with the temperature increase, causes diffusive bonding of the cladding to the substrate. Diffusive bonding is accomplished by holding the two metals to be joined in intimate contact, and thereafter heating the metals to a temperature which will cause diffusion of the atoms of one or both metal parts into the other. When the workpiece is composed of two parts of the same metal, the joint will be substantially undetectable. In the case of different metals the joint will generally be an alloy of the metals with a composition graduating from one to the other.
This process provides an ideal mechanism for controlling or totally eliminating porosity or voids which occur during other types of metal joining processes. The simultaneous application of heat and isostatic pressure combines to collapse voids by creep-like mechanisms or compressive plastic deformation and thereby joins the materials by diffusion bonding. The net result is improved reliability and efficiency of materials utilized.
Because of the tendency for most metals to acquire surface films of oxides and other compounds particularly when heated, the metal surfaces must be thoroughly cleaned and heating must be done in an inert gas, or in a vacuum to prevent further oxidation.
However, the hot isostatic pressing process is not without its problems. In some instances, surface-connected porosity has prevented complete diffusive bonding of the cladding to the substrate. This porosity has been found to be caused by the presence of the pressure transmitting medium in the cladding-substrate interface.
Several methods have been used to eliminate this problem, see U.S. Pat. Nos. 3,928,901 and 3,952,939 (Schilling, et al) and U.S. Pat. No. 3,815,219 (Wilson), which describe metallurgical methods of attaching a sheet cladding to a substrate by preforming the sheet cladding to the substrate and masking the seams formed therebetween. "Masking" involves the step of either tack welding or vacuum brazing the cladding sheet to the substrate along the seams. It is the function of masking the seams about the cladding-substrate interface to insure that during hot isostatic pressing the pressure transmitting medium is kept from entering this interface. Generally, a thin nickel plate is used as the braze material and is placed in this interface. The masked assembly is thereafter inserted into a deformable metal container which will collapse under pressures consistent with diffusion bonding. The volume of the container is such that the workpiece is completely immersed in a granular, densifying pressure transmitting medium. The container is then sealed and outgassed. Heretofore, only metallurgical methods of sealing the cladding-substrate seam from the energy transmitting medium during hot isostatic pressure were known.
It is an object of the present invention to provide an improved method of diffusion bonding of a cladding to a substrate.
It is another object of the invention to prevent penetration of the energy transmitting medium into the cladding-substrate interface during hot isostatic pressing without welding or brazing of the interface, through the use of an easy to use cladding-substrate seam isolation method and structure.
It is a further object to provide a mechanical means and method of isolating the cladding-substrate interface.
It is still a further object to provide a method and apparatus utilizing an interference fit to prevent migration of the pressure transmitting medium from entering the cladding substrate interface.
The present invention eliminates the expensive and time-consuming step of either tack welding or brazing of the cladding to the substrate before subjecting the workpiece to hot isostatic pressing. The method according to the present invention is carried out by preforming a cladding to a substrate and holding these materials together through the use of a seam isolation means. Thereafter, the cladding-substrate assembly is subjected to hot isostatic pressing. One form of the cladding-substrate seam isolation means is comprised of a cylindrical cup-shaped member which snugly secures the cladding-substrate assembly together and isolates the cladding-substrate seam by preventing migration of the energy transmitting medium from coming into contact with the seam and therefore into the cladding-substrate interface during hot isostatic pressing.